This invention relates generally to multilayer electronic devices used in the electronics industry.
The manufacture of small electronic components such as varistors, resistors, thermistors and the like requires numerous processing steps. Several different methods are employed to provide devices capable of storing electrical energy on a circuit board. After the microelectronic devices are constructed, manufacturers attach such devices to circuit boards at high speed using soldering techniques.
In soldering such electronic devices to circuit boards, the devices must have readily solderable end terminations. Electroplating metal upon the end terminations of such devices, without also plating metal upon the resistive or semiconductive bodies of such devices, is a challenge facing the industry. Manufacturers must apply metal upon only those portions of the device upon wherein they wish to impart conductivity, and seek to avoid unintentionally imparting metal to other poorly conductive portions of such devices. Separating conductive portions from semiconductive portions is sometimes very difficult in devices that are only a few hundredths of an inch wide.
Many microelectronic devices are so small as to be barely discernible to the human eye. Thus, it is usually not possible to apply metal plating to only certain preselected portions of the device by mechanically directing solder to such locations, as may be pursued in the manufacture of larger electronic components. Instead, other techniques are necessary to electroplate metal upon only certain predetermined portions of the electronic device, while avoiding electroplating metal upon other portions of the device that are designed to be nonconductive.
Several United States patents discuss the application of resin or polymeric coatings to relatively large, flat, silicon wafers. Further, other patents describe the application of resins to wafers by spin coating to form circuit boards. Etching is often used to remove resin from preselected portions of the wafer.
Resin coatings may be applied to the two-dimensional surface of a wafer. U.S. Pat. No. 5,879,572 (which is hereby incorporated by reference) is directed to the application of divinylsiloxane (DVS) bisbenzocyclobutene (BCB) as a coating to a silicon wafer. The method includes a subsequent step of etching the wafer to remove the coating from the surface of the wafer at the specific locations where it is desired to facilitate conductivity upon the wafer.
U.S. Pat. No. 5,882,836 (hereby incorporated by reference) describes the application of a photocurable formulation of partially polymerized DVS resin by spin coating the resin upon the relatively large, flat surface of a substrate or wafer to make a circuit board. A coating of 10-12 microns thick is applied by spinning the circuit board wafer a high rpm, followed by a soft bake cycle. Then, a pattern of light is applied to the surface of the wafer by striking the DVS resin on the surface of the wafer, forming a patterned circuit board with conductive and nonconductive areas on the surface of the board.
U.S. Pat. No. 5,854,302 (incorporated herein by reference) is directed to a process for forming a partially polymerized DVS resin comprising heating DVS monomer in a solvent at a concentration of DVS monomer to render the resin photocurable.
Application of resins, followed by selective removal of resins from only certain portions of microelectronic devices, is a difficult, time consuming and expensive procedure. Thus, in the manufacture of microelectronic devices, a device and process for applying resin that does not require selective removal from a portion of the device to form conductive and nonconductive portions is desirable. Thus, a process that is capable of applying resin to a microelectronic device on all portions of the device, but still enables a device having both conductive and nonconductive portions, without selective removal of resin, is highly desirable.
Devices to be resin coated which are only a few hundredths of an inch wide present special problems. Such devices are sometimes no larger than a grain of sand. A process that is capable of applying resin to such devices in mass production quantities, while still allowing recovery of excess resin, is desirable.
The invention of this application may be presented in many different embodiments, and representative embodiments are described herein. The invention is not limited to only those embodiments described below, and a person of skill in the art may readily apply the invention in other ways that are apparent from this specification.
The invention comprises an electronic device having a multi-sided body defined by a plurality of electrode plates arranged in a stack. Additionally, a plurality of terminal structures are electrically connected to said electrode plates in a predetermined manner, the terminal structures having interior and exterior layers. In general, the multi-sided body and the terminal structures are capable of receiving a resin coating. The body has a resin coating applied upon at least a portion of an exterior surface, the resin coating substantially preventing plating of metal onto the exterior surface of the body. Further, the terminal structures contain at least one metal plating layer on their exterior surface. A metal plating layer typically is affixed to the resin coating on the terminal structures in such a manner as to allow conductivity by displacing the surface portion of the coating during the plating process. The polymer usually remains in the termination pores and on the ceramic surface of the component.
One advantage of the resin application of this invention is that deployment of the invention may prevent undesirable plating upon the main body of the microelectronic device, while still facilitating plating upon the end terminations. Furthermore, there usually is no need for selective removal of the thermoset resin from any portion of the electronic device. The resin usually is applied to all portions of the device. Selective removal, which is difficult and costly in the manufacture of microelectronic devices of very small size, can be substantially avoided using this invention.
The electronic devices to which this invention may be applied include, but are not limited to varistors, thermistors, resistors, and other semiconductor devices. In most applications, the polymeric coating is cured by crosslinking. A polymer or resin may comprise any resin capable of curing into a resistant coating. In one embodiment of the invention, the resin may be a thermoset resin. In some applications, the resin coating comprises an aromatic cyclic compound. In one embodiment of the invention, the electronic device is a varistor. At least one layer of the metal plating applied to the end terminations may be comprised of nickel or a nickel alloy. In another embodiment, the metal plating is comprised of an alloy containing tin, such as a tin/lead alloy. In general, any metal platings commonly used in the microelectronics industry may be employed in the practice of this invention.
In one application of the invention, the resin coating applied to the overall device fills voids existing on the surface of the multi-sided body, or in the end termination glass frit. The filling of such voids increases the resistance of the microelectronic device to subsequent entrapment of moisture or other foreign substances in the voids. This can serve to increase reliability in the operation of the electronic device when it is subjected to high temperature and/or high humidity conditions. Unintended entrapment of metal or moisture into voids may cause an electronic component to rupture when it is subjected to reflow soldering and the like at relatively high temperatures. Thus, in one embodiment of the device, the terminal structures comprise void spaces that are substantially filled with thermoset resin prior to plating.
One embodiment of the invention employs a varistor comprising a multi-sided body defined by a plurality of electrode plates arranged in a stack. The varistor also includes a plurality of dielectric layers between the electrode plates. Further, a plurality of terminal structures are electrically connected to the electrode plates in a predetermined manner, the terminal structures having interior and exterior layers. The outer or exterior layers of the terminal structure also may comprise at least two layers of metal plated onto the terminal structures. The multi-sided body of the varistor, and the terminal structures, generally are capable of receiving a resin coating, the resin coating comprising a benzocyclobutene (BCB). Typically, the metal is plated over the resin coating in such a manner as to allow for conductivity by displacing the surface portion of the coating during the plating process. The polymer remains in the termination pores and on the ceramic surface of the component. The multi-sided body of the varistor may have a coating of BCB on at least a portion of an exterior surface. The BCB coating substantially prevents plating of metal onto the surface of the body, but does not interfere with plating upon the terminal structures. The terminal structures may comprise a metal plating layer of a tin alloy applied on the exterior surface and a metal plating layer of nickel applied on top of the BCB coating. The metal plating is affixed to the terminal structures, but usually is not affixed to the multi-sided body of the varistor.
In one aspect of the invention, a process of making an electronic device is shown, comprising several steps. First, a multi-sided body is provided having on its interior a plurality of stacked electrode plates. Next, one or more terminals are electrically connected to said electrode plates, the terminals having void spaces on their outer surface. This can be facilitated by vacuum impregnation to force material into the voids. N a next step, the electronic device is coated on all sides with a thermoset resin. This can be facilitated by vacuum impregnation to force material into the voids.
In one aspect of the invention, it is possible to plate multiple layers of dissimilar metals upon the terminals. Depending upon the identity of the resin employed, the process also may include crosslinking the thermoset resin during the curing step. Crosslinking may be by any means, including thermal, e-beam, plasma, or by light (ultraviolet to visible).
In one embodiment of the invention, the coating step comprises removing excess thermoset resin from the electronic devices by centrifugal force. The process also may include removing excess resin from the microelectronic devices by placing the electronic devices in a centrifuge, and applying centrifugal force.
In one aspect of the invention, the thermoset resin employed is a benzocyclobutene (BCB) thermoset resin. The process also may include filling void spaces with resin during the coating step on the outer surface of the terminals prior to plating metal upon the terminals. This may serve to prevent undesirable entrapment of moisture, salts or foreign debris into void spaces, which can cause rupture of terminations during later application of the device to a circuit board at high reflow solder temperatures. The invention also may include a surface mount device (SMD) made by the process described above.